Chromosomal Phylogeny and Karyotype Evolution in x=7 Crucifer Species (Brassicaceae)

Abstract

Karyotype evolution in species with identical chromosome number but belonging to distinct phylogenetic clades is a longstanding question of plant biology, intractable by conventional cytogenetic techniques. Here, we apply comparative chromosome painting (CCP) to reconstruct karyotype evolution in eight species with x=7 (2n=14, 28) chromosomes from six Brassicaceae tribes. CCP data allowed us to reconstruct an ancestral Proto-Calepineae Karyotype (PCK; n=7) shared by all x=7 species analyzed. The PCK has been preserved in the tribes Calepineae, Conringieae, and Noccaeeae, whereas karyotypes of Eutremeae, Isatideae, and Sisymbrieae are characterized by an additional translocation. The inferred chromosomal phylogeny provided compelling evidence for a monophyletic origin of the x=7 tribes. Moreover, chromosomal data along with previously published gene phylogenies strongly suggest the PCK to represent an ancestral karyotype of the tribe Brassiceae prior to its tribe-specific whole-genome triplication. As the PCK shares five chromosomes and conserved associations of genomic blocks with the putative Ancestral Crucifer Karyotype (n=8) of crucifer Lineage I, we propose that both karyotypes descended from a common ancestor. A tentative origin of the PCK via chromosome number reduction from n=8 to n=7 is outlined. Comparative chromosome maps of two important model species, Noccaea caerulescens and Thellungiella halophila, and complete karyotypes of two purported autotetraploid Calepineae species (2n=4x=28) were reconstructed by CCP.

Fast Diploidization in Close Mesopolyploid Relatives ofArabidopsis

“…AK-like association of GBs with one arm in inverted orientation compared with the ACK but without an active centromere was interpreted as the result of chromosome fusion through a pericentric inversion followed by reciprocal translocation involving terminal breakpoints and the loss of one of the two resulting products (the minichromosome). This mechanism has been described for several other crucifer species Schubert, 2007;Mandá ková and Lysak, 2008). This type of rearrangement accounts for 72 chromosome fusion and centromere loss events in all three karyotypes.…”

“…A similar telomere-to-telomere translocation has been assumed for an origin of the large metacentric chromosome of the ant Myrmecia pilosula (n = 1; Imai and Taylor, 1989) and of the human chromosome 2 (Ijdo et al, 1991b). In Brassicaceae, end-to-end chromosome fusions were not described previously Schubert, 2007;Mandá ková and Lysak, 2008).…”

“…However, the two parental genomes while having the ACK-like structure could already have deviated by chromosome number. Chromosome number reductions from n = 8 to n = 7 to 4 occurred repeatedly in Brassicaceae Schranz et al, 2007;Mandá ková and Lysak, 2008; this study).…”

“…Supplemental Table 1 online shows the structure and position of ancestral genomic blocks in the karyotypes of Stenopetalum nutans, Stenopetalum lineare, and Ballantinia antipoda as well as corresponding Arabidopsis BAC contigs. DNA probes were labeled with biotin-dUTP, Cy3-dUTP, digoxigenindUTP, and with diethylaminocoumarin (DEAC)-dUTP and Alexa Fluor 488-dUTP (S. nutans chromosome SN4; Figure 2B) by nick translation and ethanol precipitated (Mandá ková and Lysak, 2008). The hybridization probes and chromosomes were denatured together on a hot plate at 808C for 2 min and incubated in a moist chamber at 378C overnight or for 48 h.…”

Molecular phylogeny and systematics of the tribe Chorisporeae (Brassicaceae)

“…It is often difficult to unravel phylogeny at various levels and to create a natural classification because of substantial homoplasy in almost all morphological characters (e.g., Koch et al 2003;Al-Shehbaz et al 2006;Warwick et al 2010). However, considerable progress has recently been made both in phylogeny and systematics at the generic and tribal levels Bailey et al 2006;Beilstein et al 2006Beilstein et al , 2008Al-Shehbaz and Warwick 2007;Koch et al 2007;Warwick et al 2007;German and Al-Shehbaz 2008a;Mandáková and Lysak 2008;Koch and Al-Shehbaz 2009;Franzke et al 2009;German et al 2009; Electronic supplementary material The online version of this article (doi:10.1007/s00606-011-0452-0) contains supplementary material, which is available to authorized users. Khosravi et al 2009;Mandáková et al 2010;Couvreur et al 2010).…”